1. Technical Field of the Invention
This invention broadly relates to the reduction of ferric ions in aqueous acidic solutions. The invention also relates to the prevention of the formation of ferric iron-containing compounds in aqueous acidic compositions. This invention further relates to the treatment of a subterranean formation to stimulate the production of a fluid, such as a hydrocarbon, therefrom, wherein the treatment is conducted in the presence of iron-containing materials. This invention still further relates to the prevention of the formation of sludge in certain crude oils caused by the presence of ferric ions formed during the acid treatment of hydrocarbon-containing subterranean formations.
2. Description of the Prior Art and Problems Solved
It is well established in the prior art that the presence in aqueous acid solutions of dissolved iron in the ferric oxidation state can lead to the formation of ferric iron-containing compounds which produce insoluble iron solids when the pH of the acid solution increases to a value greater than about 4. In this regard Walker et al, in U.S. Pat. No. 4,683,954, and Dill et al, in U.S. Pat. No. 5,084,192, teach that ferric compounds, such as ferric hydroxide, begin to precipitate from hydrochloric acid solution when the pH of the acid increases to a value of about 2.5 and greater and that precipitation is complete when the solution pH is about 3.5. This precipitation phenomenon becomes a serious problem when an acid, such as hydrochloric acid, containing dissolved ferric iron is being used to react with a subsurface, acid soluble, calcareous formation, such as limestone, wherein the acid reaction causes the pH of the acid to typically spend to a value greater than about 4 or 5.
In addition to the precipitation problem discussed above, which can be caused by the presence of ferric ion in acid, it is taught by several authorities that hydrochloric acid having a high concentration, that is about 28% or more, can cause the development of sludge when the acid is placed in contact with certain types of crude oil. The sludge formation problem is exacerbated when the acid, which is in contact with the crude oil also contains ferric ion. In this connection see Houchin et al, "The Occurrence and Control of Acid-Induced Asphaltene Sludge," SPE 19410, February 1990; Dill et al, U.S. Pat. No. 5,084,192; Pachla et al, U.S. Pat. No. 5,063,997; and Ford, U.S. Pat No. 4,823,874.
For purposes of this invention, sludge is defined as a solid material formed in crude oil containing asphaltenes and maltenes which constituents may, under certain conditions, as pointed out above, precipitate from the crude oil. Sludge formed in crude oil while the crude oil is in a formation can render very difficult the task of recovery of the oil from the formation. Crude oil containing quantities of asphaltenes and maltenes subject to the production of sludge is referred to herein as heavy crude and sometimes as sludging crude.
Accordingly, the sludging problem specifically addressed herein is caused by the combination of acid, especially high concentration hydrochloric acid, and ferric ion in contact with a sludging crude. This problem is particularly severe when the sludge is produced during formation acidizing.
Formation acidizing or, simply, acidizing, is a method well known in the prior art utilized to increase the flow of fluid from a subterranean formation. According to the method, the formation is contacted with an acidic composition to react with and dissolve materials contained therein for the purpose of increasing the permeability of the formation. The flow of fluid from the formation is therefore increased because of the increase in formation permeability caused by the dissolution of the material. A known method of acidizing comprises the steps of conducting an acid composition to the formation through tubing disposed in a borehole penetrating the formation; forcing the acid composition into contact with the formation; and permitting the acid to react with and dissolve certain materials contained therein to thereby enlarge pore spaces within the formation and thus to increase the permeability thereof.
It is apparent that the object of formation acidizing, which is to increase formation permeability, can be frustrated if the very acid composition employed in the treatment to achieve the object, produces an environment which fosters the development of solid material which can fill and plug pore spaces in the formation, the consequent result of which is the failure to increase, and the possibility of even decreasing, formation permeability.
In the context of formation acidizing, ferric ion can be introduced into the acid as a result of reaction between ferric compounds, such as rust and millscale, on the interior wall of the conduit which conducts the acid to the formation, reaction of the acid with ferrous compounds in the formation followed by oxidation of ferrous ion to ferric ion and reaction between the acid and formation minerals that include ferric compounds, such as goethite, FeO(OH), magnetite, Fe.sub.3 O.sub.4, and hematite, Fe.sub.2 O.sub.3.
Solutions to the problems of precipitation of ferric iron compounds from spent acid and the formation of sludge induced by the contact between ferric ion and acid with sludging crude revolve about the control of ferric ion in acid and/or the elimination thereof from acid. Thus it has been suggested that ferric compounds should be removed from metal conduits, such as by pickling, prior to acidizing. Dill et al ('192) disclose the use of a blend of formic acid and acetic acid, in combination with anti sludge agents and iron control agents. Ford ('874) discloses the use of anti sludging agents, such as quaternary ammonium salts of fatty amines in hydrochloric acid. Crowe (U.S. Pat No. 4,574,050) discloses the use of an iron control agent, such as ascorbic acid and erythorbic acid, in hydrochloric acid. Pachla et al in U.S. Pat. No. 5,063,997 appear to disclose the reduction of ferric ion to ferrous ion in hydrochloric acid with hypophosphorous acid and a catalyst material selected from cupric and cuprous compounds
Numerous other compositions and methods are disclosed in the art for controlling ferric iron in formation acidizing procedures. However, the need for other such compositions and methods remains and is not diminished.